转换为斯威夫特3错误，可选的类型不解开

Question

此代码与斯威夫特2书面的价值的工作，但不能与斯威夫特3.

ShoppingManager.getShoppingItemsForPage(currentPage, pageSize: 40) { (shopItems: [ShopItemModel]?, error: Error?) ->() in 
     if error == nil { 

... 

self.shopItems.append(Array(shopItems[10..<40])) 

我得到一个错误

value of optional type [ShopItemModel] not unwrapped。

我试过使用?但它不起作用。

这里是我的ShopItemModel

class ShopItemModel: NSObject { 
    //RestKitMapping 
    var itemID: String? 
    var shopName: String? 

Answer 1

展开使用guard let声明否则你可以强制拆开包装使用!但如果shopItems是零

guard let shopItemArray = shopItems else { 
    return // shopItems is nil if it goes here 
} 

self.shopItems.append(Array(shopItemArray[10..<40])) 

Answer 2

这编译就会死机你shopItems：

ShoppingManager.getShoppingItemsForPage(currentPage, pageSize: 40) { (shopItems: [ShopItemModel]?, error: Error?) ->() in 
    if error == nil { 
     let lastItem = self.shopItems.count 

     if self.currentPage == 1 { 
      self.shopItems.append(contentsOf: Array(shopItems![10..<40])) 
     } else { 
      self.shopItems.append(contentsOf: Array(shopItems!)) 
     }